1. Field of the Invention
The present invention is related to controlling pixel gain of pixels associated with an imaging circuit associated with an image sensor. It is particularly directed to a Read Out Integrated Circuit (ROIC) of an image sensor.
2. Background
One aspect of image sensor systems, including infrared image sensor systems, is the dynamic range of the scene which can be sensed usably. The dynamic range is typically defined as the ratio of the maximum light or photon level which can be sensed without saturation to the lowest level which can be distinguished from the sensor intrinsic noise. A dynamic range of a few thousand to a few tens of thousand is typical for most image sensor pixels. However, this range is not sufficient for all possible uses. In particular, nighttime imaging where the scene contains bright light sources and also contains dim objects of interest obscured in shadows or other dark areas is particularly difficult to image effectively with conventional imaging systems. The problem is that while the sensor can often be adjusted over a wide range of dynamic ranges, all pixels in the typical system must simultaneously have the same dynamic range. One would prefer a system where some pixels are optimized for low photon levels, while others are optimized for higher levels, so that a wider range of objects could be imaged in the same image frame.
One solution to this high dynamic range image problem is to acquire many consecutive image frames looking at the same scene, with each frame varying in either its electrical gain, its integration or exposure time, or some other parameter which changes the sensitivity range of the image system for that particular image frame. These different image frames can then be combined into a single image, either electronically (as disclosed in U.S. Pat. No. 5,929,908, whose contents are incorporated by reference, or by software (such as PHOTOMATIX® available through www.hdrsoft.com, visited May 25, 2008, which combines multiple frames of still photography shots with differing exposure times to produce an extended dynamic range composite image). The result of either approach is a composite image having a much larger dynamic range than any of its constituent sub-image frames. This method is effective, but it reduces the rate of frame acquisition by the number of frames that are combined to create the single composite image. It also relies on the scene not changing during that time. Any object which is moving during this multiple frame acquisition will not appear in the same location in each sub-frame and thus will not be imaged correctly in the composite.
In order to have a large dynamic range in a single image frame, it is necessary to use a system where each pixel has the required dynamic range. This eliminates problems associated with having to combine consecutive frames into a composite. However, the dynamic range requirements for the pixel will generally not be realizable with a system having a linear response, due to the practical limitations of electrical circuits.
One approach to solving this problem utilizes some sort of non-linear response in the pixel, typically piece-wise linear (as disclosed in U.S. Pat. Nos. 6,040,570, 6,101,294 and 6,992,713, all of whose contents are incorporated by reference), or logarithmic as disclosed in Kavadia, S., “Logarithmic Response CMOS Image Sensors with On-Chip Calibration”, IEEE JSSC v. 35 n. 8, August 2000, pp 1146-1152. The problem with these approaches is that they are difficult to realize practically without adverse consequences for other performance aspects of the pixel.
A second approach is to reset the pixel whenever it saturates, and count the number of resets, as disclosed in Kavusi, S., “Architectures for high dynamic range, high speed image sensor readout circuits”, 2006 IFIP International Conference on Very Large Scale Integration (ISBN 3-901882-19-7), October 2006, pp. 36-41. This approach has the drawback of requiring a large area for the pixel, much larger than desired for high definition image systems.
A third approach is to provide some pixels with a low gain response, and others with a higher gain response. This avoids the practical limitations of non-linear response in every pixel, since now each pixel can be linear, and avoids the problems of having to combine sub-frames into a composite image, since the total range of dynamic range will be covered by the differing gains in the individual pixels. However, effective spatial resolution is reduced in this approach, since several adjacent pixels with different gains will have to be combined to produce the total dynamic range required.
Various methods may be used to form pixels having different response gains. For instance, the Fujifilm SR Pro camera series uses a pixel combining one photodetector with a large area for high sensitivity, and another photodetector with a small area for low gain, producing a composite pixel with an extended dynamic range. This results in adjacent pixels having different size detector responsive areas. Since the response gain is proportional to the responsive area, this produces the different gains. The various pixels are hardwired to have different size detector areas at design time, since the responsive areas are determined by the manufacturing process, and cannot be changed in use.
In a read out integrated circuit (ROIC) and other imaging circuits, the incident light or photons create an electrical current which is integrated onto a capacitor, creating a voltage which is proportional to the amount of incident photons. The proportionality depends on the size of the capacitor and also on the voltage gain of any intervening circuit. Some prior art approaches have used multiple charge integration capacitors. In fact, some line array ROICs or imagers have over five such capacitors per pixel, and some area array ROICs or imagers have two, as disclosed in Cannata, R., “Very wide dynamic range SWIR sensors for very low background applications”, Proc. SPIE, 3698 (1999), pp. 756-765. Such an approach allows the pixel gain to be changed by selection of the capacitor used. However, every pixel in the array generally has the same size capacitor selected and pixel-to-pixel selection of the differing capacitor sizes in an array is simply not done.
FIG. 1 shows an electrical schematic of a prior art ROIC pixel circuit 100. ROIC pixel circuit 100 includes an amplifier 110, and an integrating capacitor 120. The photon generated electrical current 102 is integrated into the capacitor 110, as depicted by the arrow.
FIG. 2 shows an electrical schematic of another prior art ROIC pixel circuit 200. ROIC pixel circuit 200 includes an amplifier 210 and two charge integration capacitors 220, 222, arranged in parallel. A pixel gain select switch 230 is arranged in electrical series with the second charge integration capacitor 222. During operation, charge is always accumulated into the first charge integration capacitor 220. However, charge is accumulated into the second charge integration capacitor 212 only if the pixel gain select switch 230 is closed. Closing the pixel gain select switch 230 changes the total capacitor size and thus the response gain. Typically all such pixel gain select switches 230 are connected together so that the switch in every pixel is either open or closed, causing each pixel to have the same gain. Thus, the prior art ROIC pixel 200 has a first state in which the pixel gain select switch 230 is open and charge is accumulated only in the first charge integration capacitor 200, and a second state in which the pixel gain select pixel switch 230 is closed and charge is accumulated only both the first and second charge integration capacitors 220, 222, respectively. As is known to those skilled in the art, such a switch 230 is typically implemented as a transistor.